1. Field of the Invention
The present invention relates an image forming apparatus including a fixing unit.
2. Description of the Related Art
Conventionally, a toner image formed on a recording sheet (hereinafter referred to as “recording material”) using an electrophotographic process is subjected to heat-fixing processing by a fixing unit. A majority of the recent electrophotographic toners contain a release wax as a constituent material. A release wax is added in order to provide effects such as adjustment of the glossiness of a printed image and dispersibility of pigments as well as prevention of fixing offsets.
Here, there are some types of fixing-offset phenomena as described below. In a process of fixing toner onto a recording material, if the fixing roller or the fixing film is not sufficiently heated (has a low temperature), the toner is not sufficiently fused, and the strength of fixing on the recording material is decreased. Such a state may result in a part of the toner adhering to the fixing roller. Such phenomenon is called a “cold offset”, and the part of the recording material in which the toner adheres to the fixing roller appears as a missing part of the image on the recording material. In addition, the fixed toner may fall out of the recording material due to, e.g., friction because of its weak fixing strength.
On the contrary, if the temperatures of the fixing roller and/or the fixing film are excessively high, the toner is sufficiently fused, but its viscosity is lowered and the fused toner partially falls out of the recording material, contaminating the fixing-roller surface. This phenomenon is called “hot offset”, which results in a missing part of the image appearing on the recording material as in a cold offset.
Therefore, in order to prevent the above-described fixing offsets, Japanese Patent Application Laid-Open No. H08-184992 proposes the addition of wax components to toner as a release agent. The inclusion of release wax in toner causes the release wax to move to the interface between the fused toner and the fixing roller when heat-fixing is performed, enhancing the withstanding of the offset phenomenon. Furthermore, Japanese Patent Application Laid-Open No. 2000-003070 proposes a technique in which two or more types of release waxes are added to toner for offset-resistibility enhancement.
A recording material is simultaneously heated and pressured by a fixing roller and a pressure roller, and then pinched and conveyed by the rollers, so that a release wax included in toner is liquefied. A majority of the release wax is fixed to the recording material together with the fused toner, while a part of the release wax vaporizes and enters a gaseous state.
Components of the evaporated release wax float in a liquid or fine solid particle state within the fixing unit depending on the temperature of the surroundings. In some cases, the floating release-wax components may adhere to various parts within the image forming apparatus.
The accumulation of such components on, for example, a recording-material conveyance roller adversely affects conveyance performance, causing, e.g., jamming of a recording material during conveyance or a decrease in the friction coefficient of the roller, and thus, is a significant problem to be solved for ensuring the reliability of the apparatus, as well as extending the life of the apparatus. Accordingly, it is necessary to take special measures, such as controlling the direction of the air flowing in the apparatus, in order to prevent the adherence of the release-wax components to the conveyance part.
Meanwhile, in recent years, there are very high demands for speeding up and downsizing of laser beam printers. An increase in the speed for performing heat-fixing processing in an image forming apparatus requires higher thermal energy and pressure than ever before. As the thermal energy provided to toner increases, the amount of evaporation of the release-wax increases, and thus, it can be anticipated that the aforementioned problems related to the adherence of evaporated release-wax components occur more frequently. Accordingly, it is important to develop a technique to collect evaporated components of a release wax included in toner in a heating and pressure-applying fixing unit.